.beta..sub.2 -microglobulin (.beta..sub.2 -m) is a low molecular weight protein (mol. wt. 11,800) present in small amounts in normal urine and other biological fluids. .beta..sub.2 -m is structurally related to immunoglobulin G and to the cell-surface histo-compatibility antigens. It is synthetized by all nucleated cells and is present on their surface. The biological role of .beta..sub.2 -m is still unknown.
In healthy subjects, the concentration of .beta..sub.2 -m in serum fluctuates within a narrow range of values, from 0.09 to 0.24 mg/100 ml. Elevated serum levels are found in malignant disorders, particularly in multiple myeloma and monocytic leukemia, and in patients with decreased glomerular filtration rate. In the latter case, the serum .beta..sub.2 -m is considered as a very sensitive index of impaired glomerular filtration rate. In healthy subjects, the amount of .beta..sub.2 -m excreted in urine is small (70-80 .mu.g/24 hr.). The urinary excretion of .beta..sub.2 -m is considerably increased in case of tubular dysfunction as observed in chronic cadmium poisoning, Fanconi's syndrome, Balkan nephropathy and Wilson's disease. In occupational medicine, the determination of .beta..sub.2 -m in urine is regarded as the most sensitive test for detecting at an early stage a tubular damage induced by excessive exposure to cadmium.
Determination of .beta..sub.2 -m in human urine or serum was initially performed by single radial immunodiffusion. This method is, however, not sensitive enough for detecting .beta..sub.2 -m in normal urine without a preliminary concentration step. Radioimmunoassay (R.I.A.) methods were then developed which can accurately measure .beta..sub.2 -m concentrations in normal urine or serum without a preconcentration of the samples. The two available R.I.A. methods for .beta..sub.2 -m analysis are the solid phase R.I.A. of Evrin et al. "Radioimmunoassay of .beta..sub.2 -microglobulin in human biological fluids," Scand. J. Clin. Lab. Invest. 28, 439-443 (1971) and the double antibody method of Shuster et al. ".beta..sub.2 -microglobulin in neoplastic diseases," Clin. Chim. Acta 67, 307-313 (1976). Solid phase R.I.A. has been commercialized by Pharmacia Diagnostic A.B. (Phadebas.RTM., .beta..sub.2 -micro Test, Uppsala, Sweden). The disadvantages of the radioisotopic methods (short shelf life of the reagents, high cost of the kits, health hazards, time consuming technique) have lead us to develop an alternative method more convenient for routine evaluation of .beta..sub.2 -m.
A lymphocytotoxicity inhibition technique for determining .beta..sub.2 -m has recently been proposed but this method is much less accurate and sensitive than R.I.A. and can give only a rough estimate of the .beta..sub.2 -m level. According to the present invention, we propose a new highly sensitive method which does not require the use of radioisotopes. This method called latex immunoassay offers at least the same precision, specificity and sensitivity as the R.I.A., but presents the advantage of being much more simple and rapid.